Much electrical and electronic equipment requires A.C. power of relatively constant and precise characteristics in order to function properly. For such sensitive equipment any material change in frequency or voltage from certain lower and upper limits or momentary or brief complete power interruptions can result in serious misfunctioning or malfunctioning, and losses and damage to either the sensitive equipment or its work, or both, can occur. Computers, for example, can lose their programs or fail to properly complete the work being processed, and have memory failures, and electrical machines operated by computers can be harmed and/or the materials being worked on damaged or destroyed, due to even brief A.C. power failures of only a few cycles and power line faults of as little as a 20% undervoltage or overvoltage and high voltage spikes, or a change in frequency of as little as a half cycle per second.
Most electrical and electronic equipment is powered by public utility A.C. power lines and, normally, these utility power lines are quite reliable in delivering A.C. at specified voltages and frequency. However, it is well known that utility A.C. power lines are subject to numerous minor faults, often numbering several thousand a year, which faults may involve substantial voltage variations, both under- and overvoltages, frequent brief outages lasting from a few cycles to a few seconds, and occasional major A.C. faults and complete power interruptions for substantial periods of time, which may extend from several minutes to one or more days at a time. Extremes of weather, fires, accidents, and similar occurences together with the electric utility's normal operating procedures under such circumstances, can result in major reductions of voltages known as "brownouts", or lengthy complete interruptions. In the typical utility system less than 5% if these irregularities and disturbances last for more than 10 to 15 seconds. In one recent study of a major public utility, covering a 4 month period in which, over and beyond many minor line irregularities, slightly over 100 power line problems capable of materially affecting computers were noted: in this period the utility line had 8 total outages of under 0.5 seconds, 7 outages of from 0.5 to 1 second, 5 outages lasting from 1 to 30 seconds, and 3 were over 30 seconds. Other utility lines may have more than this number and pattern of power line irregularities. Generally, from 1% to 5% of the material irregularities exceed 30 seconds.
In order to protect sensitive electrical and electronic equipment against A.C. power supply irregularities, there have been suggested a variety of protective devices for interposition between the A.C. power supply and such equipment. Some of such protective devices are designed for minimal protection at a low cost and comprise spike suppressors and line filters, and the like. For greater protection more costly devices such as isolation transformers and voltage regulators can be employed. For protection against voltage spikes and similar voltage and frequency irregularities and also for supplying power during brief power interruptions of from about 1/10 second to a maximum of about 1/3 second, more costly motor generator sets are used. However, the available motor-generator sets, which normally must operate continuously, have relatively low overall electrical efficiency, being in the range of from about 65% to 75% for 10 to 20 KW units and with only slightly higher efficiencies for higher capacity units of 50 to 100 KW. Further, their "ride through" time of less than about 1/3 of a second is not sufficient to supply the load in the many cases where the interruptions last from about 1/3 of a second and up to 15 to 60 seconds, nor do they allow sufficient time to transfer the load to a stand-by engine-generator set. For the most complete protection, including power interruptions lasting for several seconds and up to several hours, there are often used complex and expensive systems comprising a set of storage batteries coupled with electrical converters, and an array of electronic controls, and battery chargers. These battery based systems require a separate battery room, need frequent and considerable maintenance and in many cases replacement of the batteries is needed after only 3 to 4 years of use.
Where the electrical devices and equipment must be supplied with electrical power at all times, as in hospitals and police stations, it is usually necessary to provide back-up systems to the normal utility line hook-up, comprising an auxiliary gasoline or diesel engine driven electrical generator unit that is put into operation when the A.C. power line fails completely. It takes some few seconds, typically from 5 to 10 seconds, for a properly maintained engine-generator unit to start and begin delivering power when complete A.C. utility line interruptions occur. During these few seconds the computers and other sensitive equipment coupled to or controlled by the computer will often undergo program, memory and work loss, or actual equipment damage will occur.
To take care of up to some 98% of power line irregularities and faults, it is highly desirable to have available a relatively low cost, non-interruptible power system (NIPS) unit which will take the normal A.C. power from a utility line or other A.C. source, and will generate A.C. of a selected substantially constant frequency and voltage which is delivered to a sensitive load regardless of any supply line irregularities, including complete power interruptions lasting from at least about 15 seconds to a minute, such NIPS unit being electrically efficient, easy and inexpensive to use and to maintain, and is highly reliable. Such 15 seconds to a minute ride-through time period allows the operators of the sensitive equipment to cycle it down safely, and to safely handle or remove the work being processed in robotic devices or computer controlled apparatus, and thereby prevent or minimize damage to the apparatus and/or the work. The present invention will provide a NIPS unit having these desired characteristics. Where continued A.C. supply to the load is required at all times the ride-through time of such a NIPS is sufficient to allow an auxiliary engine-generator to be put into operation to supply the NIPS motor with A.C. so that it will continue to generate for the sensitive load the desired A.C. without a break.